The present invention relates generally, as indicated, to anti-skid brake control systems for vehicles having tandemly spaced axles and, more particularly, to such a system that combines the advantages of an axle-by-axle monitoring technique with common controller circuitry and valving.
Anti-skid brake control systems are shown in U.S. patent applications Ser. No. 685,267, filed May. 11, 1976, now U.S. Pat. No. 4,040,676, issued Aug. 9, 1977 for "Anti-Skid Brake Control System With Short Circuit Protection"; Ser. No. 769,255, filed Feb. 16, 1977 for "Anti-Skid Brake Control System With Power-Up Delay"; Ser. No. 779,205, filed Mar. 18, 1977, for "Capacitive Shunt To Minimize Noise Effects In An Anti-Skid Brake Control System"; and Ser. No. 770,535, filed Feb. 22, 1977, for "Anti-Skid Brake Control System With Circuitry for Monitoring Slower Wheel." All of these applications and the present application are co-pending and commonly assigned. As is disclosed in those applications, it is the principal purpose of an anti-skid brake control system to provide automatic overriding control of the brakes of a vehicle when an incipient or actual skid or a locked wheel condition exists. Upon detecting such condition, the system dumps part or all (hereinafter a percentage) of the brake operating fluid pressure (hereinafter air pressure) which the vehicle operator then may be requesting or attempting to apply manually by pressing on the vehicle brake pedal with his foot.
In past systems a conventional transducer detected the rotational speed of a vehicle wheel and produced an AC transducer signal having a frequency representative of the wheel speed. A frequency to voltage converter produced a DC voltage having an amplitude indicative of the frequency of the transducer signal and, thus, representative of the wheel speed. A deceleration detector circuit, such as a conventional differentiating circuit, detected the rate of downward change of the DC voltage, as an indication of the magnitude of wheel deceleration, and if that deceleration exceeded a predetermined threshhold amount indicative of an incipient or actual skid condition of the wheel, the system was operative to dump a percentage of the air pressure. If a wheel were to lock, for example, whereupon there would be no further deceleration thereof, a wheel lock detector detected the same and continued the pressure dump.
In some early systems a separate transducer, frequency to voltage converter, deceleration detector, wheel lock detector, output circuitry that determined the magnitude or percentage of the pressure dump, air brake modulator valve mechanism, and associated plumbing connections were required to provide anti-skid brake control function for each individual wheel of the vehicle. However, as is disclosed in the last application mentioned above, the system may operate on an axle by axle basis using a single electronic controller to monitor the deceleration of the more slowly rotating one of at least two wheels. In the latter anti-skid brake control system separate transducers and frequency to voltage converters were required for each wheel; but only single deceleration detector, wheel lock detector, and output circuits were required in the controller, and only one valve was necessary for both wheels on the given axle.
Moreover, as is also disclosed in the last application mentioned above, the output circuitry may include a comparator that monitors the magnitudes of the deceleration signal from the deceleration circuit and the output signal from the wheel lock detector circuit and depending on such magnitudes determines whether an also included output circuit energizes one, the other, or both of a pair of solenoids in the air brake valve to modulate the air pressure, i.e. to dump different respective percentages of the air pressure then requested by the operator. One particular type of suitable air brake valve is disclosed in a brochure entitled "Triple Action Skid Control", published by B. F. Goodrich Company, March, 1975. Such valve includes a pair of solenoids that may be selectively energized by the system to dump one third, two thirds, or all of the air pressure being requested by the vehicle operator. When neither solenoid is energized, all of the requested air pressure is delivered to the air brakes. A safe direction failure control circuit having one or more inputs coupled to respective portions of the anti-skid brake control system, for example, as is disclosed in the above-mentioned applications, monitors those portions of the system and disables the latter when a fault or the like is sensed, thus returning full manual control of the brakes to the vehicle operator.
In the past, a deceleration summing technique was used in an anti-skid brake control system to improve the stop distance efficiency of the vehicle without loss of stability when the vehicle was riding on a surface having split mu conditions.
Other anti-skid brake control systems are disclosed in U.S. Pat. Nos. 3,917,359, which employs conventional magnetic pickups or transducers for generating signals representing the rotational speed of the wheels of the vehicle, and 3,847,446, which is directed to a system for tandem axle vehicles. In the latter patent, a pair of tandemly spaced wheels on one side of the vehicle are simultaneously monitored and controlled, i.e. have dumped a percentage of the air pressure requested by driver when an incipient or actual skid is detected. A similar anti-skid brake control system is provided for the pair of tandemly spaced wheels on the opposite side of the vehicle. Of the several embodiments shown in such patent, including those in which both wheels must skid to effect a pressure dump, only the forwardly located wheel must skid to effect a pressure dump, or either wheel must skid to effect a pressure dump, control is effected only on a side-by-side basis of the vehicle. Moreover, in the majority of embodiments in such patent in which the speeds of both wheels on one side of the vehicle are monitored simultaneously, separate deceleration detector circuits are required for each wheel; and in one embodiment signals representing the speeds of two wheels on separate axles on one side of the vehicle are averaged and a single deceleration detector circuit detects the magnitude of the deceleration of the average speed of the pair of wheels.